Operational transconductance amplifiers (OTAs) are commonly used to realize certain passive components, e.g., inductors, in integrated circuit devices. In one implementation, cross-coupled OTAs are configured to constitute a gyrator. As a result of gyrator operation, a capacitance coupled across an output port of the gyrator is reflected as an inductance across the gyrator input port. The magnitude of the reflected inductance is proportional to the capacitance of the capacitor and is inversely proportional to the square of the transconductance of the OTAs that constitute the gyrator. In general, this technique is effective to instantiate components, such as inductors, that are not easily implemented through conventional integrated circuit fabrication techniques. Additionally, OTA implementation enables frequency-selective passive components (e.g., integrated inductors) to be electronically tuned by controlling the transconductance of the gyrator OTAs
However, the finite bandwidth of the gyrator OTAs is manifest as a transconductance that is a function of the frequency of operation. That is, the OTA transconductance (Gm) is bandwidth limited in a manner that is defined by the cutoff frequency of the OTA. Consequently, when the gyrator-synthesized inductance is incorporated into a filter circuit, the frequency-dependent transconductance of the gyrator causes spurious peaks in the filter passband, and causes degradation in the stopband attenuation. Conventional approaches to the remediation of these anomalies have been predominately directed to increasing the bandwidth of the gyrator OTAs. However, only limited success is achievable in this manner, in large part because OTAs with the requisite bandwidth are difficult to design and implement. Consider, for example, an active bandpass filter designed to have a cutoff frequency at 80 MHz. Simulation results suggest that in order to suppress passband peaks in the transfer function of such an OTA-implemented filter to less than 0.2 db, for example, an OTA bandwidth of 8 GHz may be required. That is to say, in such applications the OTA bandwidth is preferably at least 100 times the filter cutoff frequency. Attempts to extend the bandwidth of the OTAs to such a frequency may be confronted by the limitations of state-of-the-art semiconductor device fabrication techniques. In addition, extremely high bandwidth may sometimes be had only at the risk of instability.
Accordingly, what is desired is a technique that mitigates anomalies in the frequency-dependent characteristics of integrated circuit components that are predicated on finite-bandwidth OTAs. In particular, it is desired that there be provided a technique to minimize spurious peaks, or ripples, in the passband of Gm-C filters.